Dielectric ceramic composition and composite ceramic structure

ABSTRACT

Strontium titanate (SrTiO 3 ) and barium zirconate (BaZrO 3 ) are made into a solid solution at a predetermined ratio. Specifically, a dielectric ceramic composition is represented by a basic composition (SrTiO 3 ) (1-x) (BaZrO 3 ) x  (in the formula, X satisfies 0.63≦X≦0.95). More preferably, X satisfies 0.67≦X≦0.90 in this range. Such a dielectric ceramic composition may be integrated with alumina to form a composite ceramic structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dielectric ceramic composition and acomposite ceramic structure.

2. Description of the Related Art

Devices by utilizing creeping discharge, e.g., a creeping dischargeozonizer, have been known previously. For example, Patent Literature 1discloses a creeping discharge element in which a linear dischargeelectrode is disposed on an inside surface of alumina ceramics(insulator) formed into a cylindrical shape and a sheet-shaped inductionelectrode is disposed in the inside or on an outside surface thereof.Also, a creeping discharge type ozonizer which generates ozone byapplying a high-frequency high voltage to such a creeping dischargeelement and, thereby, generating creeping discharge is disclosed.

CITATION LIST Patent Literature

PTL 1: JP 2012-111666 A

SUMMARY OF THE INVENTION

Meanwhile, as for the device by utilizing such creeping discharge, useof various materials other than alumina has been studied. Creepingdischarge is generated by applying a high-frequency high voltage.Therefore, an insulator between the discharge electrode and theinduction electrode has been desired to have a high withstand voltagewhich can endure dielectric breakdown.

The present invention has been made to solve the above-describedproblems, and a main object is to provide a material having a highwithstand voltage.

The present inventors conducted intensive research to achieve theabove-described main object. As a result, it was found that in the casewhere strontium titanate (SrTiO₃) and barium zirconate (BaZrO₃) weremade into a solid solution at a predetermined ratio, the withstandvoltage was able to be increased as compared with the case where theywere used independently, and the present invention has been completed.

That is, a dielectric ceramic composition according to the presentinvention is represented by a basic composition(SrTiO₃)_((1-x))(BaZrO₃)_(x) (in the formula, X satisfies 0.63≦X≦0.95).

According to the dielectric ceramic composition of the presentinvention, a material having a high withstand voltage can be provided.Although the reasons such effects are obtained are not certain, it isestimated that the crystal grain sizes of the fired body become smalland uniform by formation of solid solution of strontium titanate andbarium zirconate at an appropriate ratio.

In the composite ceramic structure according to the present invention,the above-described dielectric ceramic composition and alumina areintegrated.

In the composite ceramic structure according to the present invention, adifference in thermal expansion coefficient between the above-describeddielectric ceramic composition and alumina is small, so that even when atemperature change is applied, cracking and the like resulting from thedifference in the thermal expansion coefficient do not occur easily.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the value of X andthe withstand voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Dielectric Ceramic Composition]

A dielectric ceramic composition according to the present invention isrepresented by a basic composition (SrTiO₃)_((1-x))(BaZrO₃)_(x) and ispreferably a solid solution of SrTiO₃ and BaZrO₃. In the formula, Xsatisfies 0.63≦X≦0.95. Such materials can obtain high withstand voltagesof, for example, 30 kV/mm or more. Among them, the lower limit value ofX is preferably 0.67, and more preferably 0.75. The upper limit value ispreferably 0.90, and more preferably 0.85. This is because the withstandvoltage can be further increased in such a range.

The dielectric ceramic composition has an average crystal grain size ofpreferably 4 μm or less, and more preferably 1.9 μm or less. This isbecause the withstand voltage of such a material can be increased. Here,the average crystal grain size is specified to be a value determined asdescribed below. Initially, any surface of the dielectric ceramiccomposition is polished, the polished surface is observed with ascanning electron microscope (SEM) to obtain a SEM image. Subsequently,a straight line is drawn on the resulting SEM image, and the number ofcrystals on the straight line is counted. Then, a value obtained bydividing the length of the straight line by the number of crystals isspecified to be the average crystal grain size.

The dielectric ceramic composition has a relative density of preferably97% or more, and more preferably 98% or more. This is because such amaterial is sufficiently dense and, therefore, the withstand voltage canbe increased.

The withstand voltage of the dielectric ceramic composition ispreferably 30 kV/mm or more, and more preferably 39 kV/mm or more. Thereason is that a higher withstand voltage is preferable becausedielectric breakdown of the dielectric ceramic composition can besuppressed. Also, the lower limit value of the relative dielectricconstant of the dielectric ceramic composition is preferably 44, andmore preferably 69. The upper limit value is preferably 180, and morepreferably 120. This is because the compatibility between a highrelative dielectric constant and a high withstand voltage can be ensuredin such a range. Meanwhile, the lower limit value of the thermalexpansion coefficient of the dielectric ceramic composition ispreferably 7.7 ppm/K, and more preferably 8.0 ppm/K. The upper limitvalue is preferably 9.0 ppm/K, and more preferably 8.8 ppm/K. This isbecause the thermal expansion coefficient is close to the thermalexpansion coefficient of alumina, so that integration with alumina isfacilitated, and a high withstand voltage is obtained in such a range.

The dielectric ceramic composition may be produced through, for example,(a) a raw material mixing step, (b) a forming step, and (c) a firingstep.

(a) Raw Material Mixing Step

In the raw material mixing step, a mixed raw material is obtained bymixing a Sr source, a Ti source, a Ba source and a Zr source in such away that a predetermined ratio in the basic composition(SrTiO₃)_((1-x))(BaZrO₃)_(x) (in the formula, X satisfies 0.63≦X≦0.95)is ensured. The Sr source, the Ti source, the Ba source and the Zrsource are not specifically limited and may be metal simple substances,alloys containing at least one of them, or oxides, hydroxides,carbonates, nitrates, sulfates, and the like containing at least one ofthem. Most of all, it is preferable that SrTiO₃ be used as the Sr sourceand the Ti source and BaZrO₃ be used as the Ba source and the Zr sourcebecause the dielectric ceramic composition represented by the basiccomposition (SrTiO₃)_((1-x))(BaZrO₃)_(x) can be obtained relativelyeasily. The mixing method is not specifically limited and may be drymixing or be wet mixing in the presence of a solvent. At this time, itis preferable to perform milling and mixing by using a mortar, ballmill, or the like. This is because the raw materials are mixed morehomogeneously, so that a dielectric ceramic composition in which thewhole microstructure is homogeneous is obtained.

(b) Forming Step

In the forming step, the mixed raw material obtained in the raw materialmixing step is formed into a predetermined shape to obtain a compact.Examples of forming methods include uniaxial press, isostatic press,extrusion, and injection forming.

(c) Firing Step

In the firing step, the compact obtained in the forming step is fired soas to be densified. The firing atmosphere may be an oxidizingatmosphere, a reducing atmosphere, an inert atmosphere, or a reducedpressure atmosphere, although the oxidizing atmosphere is preferablefrom the viewpoint of production of a metal composite oxide. Examples ofoxidizing atmospheres include an air atmosphere and an oxygenatmosphere. The firing temperature is preferably 1,400° C. or higher and1,600° C. or lower. In the case of 1,400° C. or higher, densificationproceeds sufficiently and the relative density increases, so that thewithstand voltage and the relative dielectric constant of the dielectricceramic composition can be further increased. Also, in the case of1,600° C. or lower, crystal grains do not become too large, so that thewithstand voltage can be further increased. In this regard,pressurization may be performed in the firing.

As for the above-described dielectric ceramic composition according tothe present invention, the withstand voltage can be increased. Althoughthe reasons such effects are obtained are not certain, it is estimatedthat the crystal grain sizes of the fired body become small and uniformby formation of solid solution of strontium titanate and bariumzirconate at an appropriate ratio.

Also, this dielectric ceramic composition has a relatively high relativedielectric constant. Consequently, the discharge start voltage of thecreeping discharge can be decreased and the power consumption can bereduced.

In addition, this dielectric ceramic composition has a thermal expansioncoefficient close to that of alumina. In the case where this dielectricceramic composition and alumina are integrated, a difference in thethermal expansion coefficient between the two is small. Therefore,cracking and the like resulting from the difference in the thermalexpansion coefficient do not occur easily even in the case where atemperature change is applied, so that high bonding strength isobtained.

In addition, this dielectric ceramic composition is densified atrelatively low temperatures of, for example, 1,600° C. or lower.Therefore, production is easily performed.

[Composite Ceramic Structure]

The ceramic structure according to the present invention is produced byintegrating the above-described dielectric ceramic composition andalumina (Al₂O₃). The method for integration is not specifically limited.The dielectric ceramic composition and the alumina may be directlybonded or be bonded with a bonding agent therebetween.

Examples of methods for direct bonding include a method in which adielectric ceramic composition before firing and alumina before firingare bonded through co-firing. Specifically, for example, an integratedcompact may be produced by bringing a compact of dielectric ceramiccomposition before firing into contact with an alumina compact beforefiring in the above-described “(b) Firing step” and bonding may beperformed by firing the integrated compact in “(c) Firing step”. Thetemperature of 1,400° C. or higher and 1,600° C. or lower suitable forfiring of the dielectric ceramic composition is also suitable for firingof the alumina and bonding of the dielectric ceramic composition and thealumina. Therefore, integration can be performed through co-firing.

Examples of methods for bonding through the bonding agent include amethod in which the bonding agent is placed on a bonding surface betweenthe dielectric ceramic composition after firing and the alumina afterfiring and the bonding agent is softened or melted by heating at leastthe periphery of the bonding agent to perform bonding. It is preferablethat a difference in the thermal expansion coefficient of the bondingagent and those of the dielectric ceramic composition and the alumina besmall. This is because cracking and the like resulting from thedifference in the thermal expansion coefficient between the bondingagent and the dielectric ceramic composition or the alumina do not occureasily even in the case where a temperature change is applied. Also, itis preferable that bonding be possible at temperatures lower than orequal to the firing temperatures of the dielectric ceramic compositionand the alumina. As for such a bonding agent, glass frit, glass paste,and the like can be favorably used.

In the above-described composite ceramic structure according to thepresent invention, a difference in the thermal expansion coefficientbetween the above-described dielectric ceramic composition and thealumina is small and, therefore, cracking and the like resulting fromthe difference in the thermal expansion coefficient do not occur easilyeven in the case where a temperature change is applied. Meanwhile, asfor such a composite ceramic structure, high efficiency creepingdischarge can be performed in the portion of the dielectric ceramiccomposition while a high insulating property is maintained in theportion of the alumina because of integration with the aluminaexhibiting a high insulating property.

In this regard, the present invention is not limited to theabove-described embodiments and can be executed in various forms withinthe technical scope of the present invention, as a matter of course.

EXAMPLES

The specific examples of production of the dielectric ceramiccomposition will be described below as experimental examples. In thisregard, Experimental examples 3 to 9 correspond to examples of thepresent invention, and Experimental examples 1, 2 and 10 correspond tothe comparative examples.

[Production of Dielectric Ceramic Composition (Fired Body)]

Strontium titanate (SrTiO₃, produced by Fuji Titanium Industry Co.,Ltd., purity ≧97%) and barium zirconate (BaZrO₃, produced by KojundoChemical Laboratory Co., Ltd., purity ≧98%) were prepared as rawmaterials. The prepared strontium titanate and barium zirconate wereweighed and mixed at the molar ratio shown in Table 1 to obtain a mixedraw material. Subsequently, the mixed raw material was added toisopropyl alcohol (IPA) serving as a solvent, and wet mixing wasperformed with a ball mill by utilizing ZrO₂ cobble stones having adiameter of 10 mm. The resulting powder was passed through a #100 sieveand was dried for a night with a nitrogen drier. A compact was obtainedby molding 100 g of the thus obtained dried powder into the shape havinga diameter of 65 mm at a pressing pressure of 100 kg/cm². The resultingcompact was further subjected to a CIP treatment (isostatic pressingtreatment) under the condition of the pressing pressure of 3,000 kg/cm²for 30 seconds. Finally, firing was performed in the air atmosphere at1,500° C. for 2 hours, so that fired bodies of Experimental examples 1to 10 were obtained.

TABLE 1 Composition Average Thermal Relative Presence or (molar ratio)Relative crystal grain Expansion Dielectric Withstand Absence of SrTiO₃BaZrO₃ Density size Coefficient Constant Voltage Crack in (1 − X) (X) %μm ppm/K — kV/mm Bonding Experimental 0.50 0.50 97.7 9.2 9.6 250 29Presence Example 1 Experimental 0.40 0.60 97.9 5.4 9.3 190 28 PresenceExample 2 Experimental 0.37 0.63 98.1 4.0 9.0 180 33 Absence Example 3Experimental 0.25 0.75 98.2 1.9 8.8 120 39 Absence Example 4Experimental 0.21 0.79 98.5 1.8 8.5 96 42 Absence Example 5 Experimental0.20 0.80 98.7 0.8 8.5 94 43 Absence Example 6 Experimental 0.15 0.8598.0 0.6 8.0 69 39 Absence Example 7 Experimental 0.10 0.90 97.7 0.5 8.054 36 Absence Example 8 Experimental 0.05 0.95 97.2 0.4 7.7 44 32Absence Example 9 Experimental 0.03 0.97 83.1 0.3 7.6 40 18 AbsenceExample 10

[Derivation of Relative Density]

The bulk density of each fired body was measured by the Archimedes'method, where a medium was pure water, in conformity with JIS-R1634.Also, the pulverized sample obtained by pulverizing each fired body witha mortar was used and the true density of each fired body was measuredwith a dry automatic densimeter (AccuPic 1330 produced by Micrometrics).The relative density was derived by dividing the measured bulk densityby the measured true density.

[Derivation of Average Crystal Grain Size]

Initially, the surface of each fired body was polished. Subsequently,the polished surface was observed with a scanning electron microscope(SEM) and a SEM image was obtained. A straight line was drawn on theresulting image, the number of crystals on the straight line wascounted, the length of the straight line was divided by the number ofcrystals and, thereby, the average crystal grain size was derived. Inthis regard, the SEM observation was performed by using XL30 produced byPhilips under the condition of the acceleration voltage of 20 kV, thespot size of 4.0, and the magnification of 10,000 times. The number ofview fields observed was arbitrarily selected five view fields per firedbody.

[Measurement of Thermal Expansion Coefficient]

Initially, a rectangular test piece of 3×3×20 mm was cut from each firedbody. Subsequently, the test piece was set into a vertical thermaldilatometer (Thermo plus EVO TMA8310, produced by Rigaku Corporation)and the thermal expansion coefficient was measured. The measurement wasperformed in a rage of 40° C. or higher and 1,000° C. or lower, wherethe reference substance was specified to be alumina and the temperaturewas raised at 10° C./min. The thermal expansion coefficient wasspecified to be the value at 1,000° C., where the reference temperaturewas specified to be 40° C.

[Measurement of Relative Dielectric Constant]

Initially, a disk-shaped test piece of 50 mmφ×1 mm was cut from eachfired body. Subsequently, the measurement was performed in conformitywith JIS-C2141 by using an impedance analyzer (6440B, produced by WayneKerr Electronics).

[Measurement of Withstand Voltage]

Initially, a test piece, which was a disk of 50 mmφ×10 mm with a surfaceprovided with a dent of 10 mmφ, was cut from each fired body.Subsequently, the withstand voltage was measured in conformity with theinternational standard IEC60672-2. In this regard, the test piece afterthe measurement of the withstand voltage had a hole due to dielectricbreakdown.

[Evaluation of Presence or Absence of Crack in Bonding with Alumina]

Initially, a rectangular test piece of 10×10×1 mm was cut from eachfired body. Also, an alumina dense body was prepared and was cut into arectangular shape of 10×10×1 mm. The two were bonded together with glassfrit (thermal expansion coefficient 8 ppm/K) therebetween, and firingwas performed in the air atmosphere at 950° C. for 2 hours. After thefiring, whether a crack was present in the glass layer or the bondinginterface between the glass layer and the test piece or not was examinedwith an optical microscope.

[Experimental Result]

Table 1 shows the composition, the relative density, the average crystalgrain size, the thermal expansion coefficient, the relative dielectricconstant, the withstand voltage, and presence or absence of crack inbonding of each of Experimental examples 1 to 10. FIG. 1 shows therelationship between the value of X and the withstand voltage. As isclear from Table 1 and FIG. 1, in the case where X in the basiccomposition (SrTiO₃)_((1-x))(BaZrO₃)_(x) satisfied 0.63≦X≦0.95, a highwithstand voltage of more than 30 kV/mm was obtained. It was found that,among them, those satisfying 0.67≦X≦0.90 were preferable because thewithstand voltages were 36 kV/mm or more and those satisfying0.75≦X≦0.85 were more preferable because the withstand voltage were 39kV/mm or more.

In all the cases where X satisfied X≦0.95, the relative densities werelarge values of 97.2% or more. Consequently, it was found that in theexamples according to the present invention satisfying X≦0.95,sufficient densification occurred even when firing was performed at arelatively low temperature of 1,500° C.

The average crystal grain size became smaller as the value of Xincreased. Here, it is considered that as the average crystal grain sizebecomes small, there are tendencies of variations in the crystal grainsizes to decrease and the withstand voltage to increase. However, inExperimental example 10 in which the average crystal grain size was thesmallest and was 0.3 μm, the withstand voltage was low and was 18 kV/mm.The reason for this was considered that in Experimental example 10,densification was not sufficient, many pores were present and, thereby,pores and dielectric interfaces on which the electric field wasconcentrated increased. Consequently, it was estimated that in theexample in which X satisfied 0.63≦X≦0.95, a high withstand voltage wasexhibited because the crystal grain sizes were small and uniform.

The thermal expansion coefficient decreased as the value of X increased.It was found that among them, the thermal expansion coefficient withinthe range of 7.7 ppm/K or more and 9.0 ppm/K or less was preferablebecause the compatibility with a high withstand voltage was able to beensured. Also, it was found that the thermal expansion coefficientwithin such a range was the value close to the thermal expansioncoefficient of alumina and was preferable because cracking and the likeresulting from a difference in the thermal expansion coefficient did notoccur easily in the case of use based on integration with the alumina.This was ascertained from the evaluation of presence or absence of crackin bonding with alumina, where cracking occurred in those having athermal expansion coefficient of larger than 9.0 ppm/K, althoughcracking did not occur in those having a thermal expansion coefficientof 7.6 ppm/K or more and 9.0 ppm/K or less.

The relative dielectric constant decreased as the value of X increased.It was found that among them, the relative dielectric constant withinthe range of 44 or more and 180 or less was preferable because thecompatibility between a high relative dielectric constant and a highwithstand voltage was able to be ensured.

The present invention claims priority to Japanese Patent Application No.2013-063608, filed in the Japan Patent Office on Mar. 26, 2013, theentire contents of which are incorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention can be utilized in, for example, the technicalfield in which devices by utilizing creeping discharge are produced.

What is claimed is:
 1. A dielectric ceramic composition represented by abasic composition (SrTiO₃)_((1-x))(BaZrO₃)_(x) (in the formula, Xsatisfies 0.63≦X≦0.95).
 2. The dielectric ceramic composition accordingto claim 1, being a solid solution of SrTiO₃ and BaZrO₃.
 3. Thedielectric ceramic composition according to claim 1, wherein the Xsatisfies 0.67≦X≦0.90.
 4. The dielectric ceramic composition accordingto claim 1, having an average crystal grain size of 4μm or less.
 5. Thedielectric ceramic composition according to claim 1, having a relativedensity of 97% or more.
 6. The dielectric ceramic composition accordingto claim 1, having a withstand voltage of 30 kV/mm or more.
 7. Thedielectric ceramic composition according to claim 1, having a relativedielectric constant of 44 or more and 180 or less.
 8. The dielectricceramic composition according to claim 1, having a thermal expansioncoefficient of 7.7 ppm/K or more and 9.0 ppm/K or less.
 9. A compositeceramic structure wherein the dielectric ceramic composition accordingto claim 1 and alumina are integrated.